The present invention relates generally to sound reproduction systems, and more particularly to multiple channel sound reproduction systems. Specifically, the present invention provides an improved stereophonic sound reproduction system that generates for listeners an improved stereophonic image within a large listening area.
A perennial problem in the design of loudspeaker systems for stereophonic sound reproduction has been the general inability to provide coherent stereo imaging within a large listening area. Numerous techniques have been developed for enhancing the image, but the use of such techniques has invariably resulted in a reduced listening area size. The trade-off has been that the sharper stereo image gained by the use of these techniques has been at the expense of having to settle for a narrower stereo listening area. Such techniques are numerous and generally include the need to make geometric assumptions which only hold true within a very narrow listening area. For example, multiple driver arrays have been configured so that the voice coils of each driver are vertically coplanar on a plane perpendicular to the listener. This effectively reduces phase shifting across the frequency spectrum since each voice coil of the array is equidistant from the listener. Unfortunately, this enhanced phase alignment is geometrically dependent upon the location of the listener, and, in a two-speaker system, can occur ideally at only one point in space.
Another technique by which stereo imaging has been enhanced has been the cancellation of interaural crosstalk. Interaural crosstalk occurs when, for example, left channel sound enters the listener's right ear and vice versa. This results in false imaging and an unnatural sense to the listener which would not be present at a live performance. Interaural crosstalk has been cancelled by inverting and delaying each signal channel and adding it to the other signal channel. The amount of delay is critical and depends, among other things, on the location of the listener with respect to the loudspeakers and the width of his head from ear to ear. Again, critical geometrical relationships are necessary for the listener to perceive the enhanced image. This results in a very small listening area.